This invention relates to a positive temperature coefficient thermistor (hereinafter referred to as "PTC thermistor"), and more particularly to an electrode structure of a PTC thermistor.
Conventionally, electroless Ni plating has been typically employed for forming an ohmic electrode on a PTC thermistor body of a PTC thermistor. A thickness of a Ni film formed by electroless Ni plating is required to be typically as large as 1 .mu.m or more and more particularly 1.0 to 5.0 .mu.m in order to establish satisfactory ohmic contact.
Also, the Ni film formed by electroless Ni plating causes an increase in contact resistance of the PTC thermistor and deterioration of the ohmic electrode with time due to oxidation when it is solely used for the purpose of forming the ohmic electrode. In order to avoid the disadvantage, a paste of Ag which is metal of low contact resistance is applied to the plated Ni film, resulting in forming a multi-electrode structure.
More particularly, the conventional multi-electrode structure for the PTC thermistor is formed by subjecting the Ag paste applied onto the plated Ni film to baking at about 500.degree. C. Unfortunately, the baking causes moisture in the thermistor body originating in a plating solution or the like to expand and burst, resulting in a number of micro-craters being formed in the plated Ni film. This leads to deterioration in appearance of the PTC thermistor to decrease the yields.
Further, in the conventional PTC thermistor, the Ni film formed by electroless Ni plating has a thickness as large as 1 .mu.m or more, so that a length of time required for the plating is disadvantageously increased. Also, this requires to use a plating equipment of an increased plating capacity and causes the amount of plating material used to be increased, leading to an increasing in manufacturing cost of the PTC thermistor:
Moreover, when a thickness of the plated Ni film is 2 .mu.m or more, the conventional PTC thermistor tends to fail to pass a Ni peeling test for determining resistance to peeling between Ni and Ag due to micro-craters in the plated Ni film. The Ni peeling test is generally carried out in a manner to apply an adhesive tape to a sample of a Ni film and then peel the tape from the sample to possibly form craters in the sample, resulting in evaluating or determining the craters.